This invention relates generally to the treatment of waste materials for the reduction and removal of hexavalent chromium from an alkaline solution, and more particularly to the treatment of sludge and similar materials in which the hexavalent chromium is reduced to trivalent chromium and then removed as a coprecipitate.
In order to meet increasingly stringent environmental protection requirements and standards, it is becoming necessary to remove or at least significantly reduce many different materials which are generated during various manufacturing processes and are present in the waste effluent in some form. In the past, waste materials were often disposed of with little thought of their content of hazardous materials, thus causing serious contamination problems at many sites and in bodies of water. Hence, increasingly rigid requirements have been set for the control of many different types of hazardous material in waste products.
One material which can cause significant problems in waste disposal is chromium. Chromium can be present in several different forms, most frequently as a compound. The most hazardous of the forms of chromium is hexavalent chromium (Cr.sup.+6). It is increasingly necessary to lower the chromium content to a few parts per million (ppm) or less in waste water or leachate. This is often done by chemically reducing the soluble hexavalent chromium (Cr.sup.+6) to trivalent chromium (Cr.sup.+3). This reduction of Cr.sup.+6 to Cr .sup.+3 does decrease the degree of hazard of the chromium to some extent. However, even the presence of Cr.sup.+3 in waste material can also cause problems and indeed there are governmental regulations which impose limits on the content of total chromium that can be discharged as effluent. Thus, it is necessary to ensure that there is minimal chromium in any form being discharged as an effluent by removing it from the liquid discharge. To this end, it is also desirable to tie up the chromium in some insoluble form such that it will not leach out to any significant extent.
There have been several prior art proposals for the reduction of Cr.sup.+6 to Cr.sup.+3. One such proposal is described in U.S. Pat. No. 4,704,259 assigned to Tenneco Canada Inc. This patent discloses the reduction of soluble Cr.sup.+6 in an aqueous chlorate solution to Cr.sup.+3 by the use of sodium dithionite and thus forming Cr.sup.+3 ions in the form of chromic hydroxide, which according to the patentee precipitates from the chlorate solution. This patent relies on the insolubility of the Cr.sup.+3 compound and thus, is dependent upon the particular end product being insoluble and filterable or otherwise removable from the solution.
U.S. Pat. No. 3,981,965 assigned to Allied Chemical Corporation discloses the use of sodium dithionite to reduce Cr.sup.+6 under alkaline conditions to treat waste residues. Again, this patent relies on the insolubility and filterability of trivalent chromium (Cr.sup.+3) hydroxide.
However, it has been found that while some forms of Cr.sup.+3 compounds may be insoluble to various degrees, nevertheless, under certain conditions Cr.sup.+3 compounds indeed are not totally insoluble or appear to form colloids which, when filtered through conventional filters, pass through with the solution and thus are not effectively removed even with filtering. Hence, any reduction of Cr.sup.+6 to Cr.sup.+3, which does not significantly tie up the reduced Cr.sup.+3 ions in a filterable precipitate, will not be completely effective in treating the waste water to a level acceptable for disposal.